The present invention refers to a surface coating of the working surface of a cylinder of a combustion engine as well as to a method of applying a surface coating to the working surface of a cylinder of a combustion engine.
Distinctive progress having been made in recent times in developing new motor oils having an extended useful life, it would be desirable to reduce the oil consumption of combustion engines to such a degree that the oil change intervals could be further extended. The objectives could be seen, for example, to change the oil only once in a 60,000 miles period without the need to top-up the oil level in the engine.
It is well known that the nature of the surface, i.e. the topography of the cylinder wall, has a crucial influence on the oil consumption. Even if a high surface finish can be achieved e.g. by honing, today""s cylinder working surfaces usually have a not closer specified porosity and are provided at least with a number of pores, respectively, which are comparatively large, thus negatively influencing the oil consumption.
The patent publication WO 99/05339 A1 discloses a thermal plasma coating process for interior walls, particularly for sleeve bearings, having as an object to avoid, whenever possible, the formation of oxides on the coating surface which is, per se, prone to oxygenation, because such oxide inclusions favor an undesired porosity. It is striven for an entire porosity of less than 3% whereby the pores shall be essentially closed. Moreover, it is suggested to roughen the applied coating to an arithmetic mean roughness Ra of 4 to 30 xcexcm. However, by the suggested measures, neither the oil consumption can be considerably lowered nor the tribologic characteristics can be considerably improved.
Further, U.S. Pat. No. 5,766,693 discloses a plasma coating method in which mixed layers consisting of metals and metal oxides in their lowest oxidation stage are created and in which the metallic regions are separated from the metal oxide regions. It is striven for a content of metal oxides of at most 30%, a degree of porosity of between 3 and 10%, a pore size of between 1 and 6 xcexcm and a surface roughness (arithmetic mean roughness) of 3.8 to 14 xcexcm (150 to 550 xcexcin). However, by the suggested measures, neither the oil consumption can be considerably lowered nor the tribologic characteristics can be considerably improved.
It is an object of the present invention to avoid the disadvantages of the prior art as discussed herein above, i.e. to provide an improved surface coating of the working surface of a cylinder of a combustion engine which offers favorable conditions for a low oil consumption and simultaneously shows good tribologic characteristics. It is a further object of the invention to provide a method for applying such a surface coating to the working surface of a cylinder of a combustion engine.
To meet these and other objects, the present invention provides, according to a first aspect, a surface coating of the working surface of a cylinder of a combustion engine, having the combination of the following characteristics:
The coating is applied by plasma spraying; the surface of the coating comprises a plurality of open pores; the degree of porosity of the surface of the coating amounts to between 0.5 and 10%; the statistic mean pore size amounts to between 1 and 50 xcexcm, whereby at least nearly exclusively pores with a size of less than 100 xcexcm are present; the pores are stochastically distributed in the surface of the coating, both as far as the area and the size is concerned; the coating comprises a content of bound oxygen of between 0.5 and 8% by weight; the coating comprises inclusions of FeO and Fe3O4 crystals, serving as solid lubricants; and the roughness of the surface of the coating is adjusted by mechanically finishing it to an arithmetic mean roughness Ra of between 0.02 and 0.4 xcexcm and to a mean peak-to-valley distance Rz of between 0.5 and 5 xcexcm.
According to a second aspect, the invention provides a method of applying a surface coating to the working surface of a cylinder of a combustion engine. Thereby, the surface coating has a plurality of open pores, the degree of porosity of the surface of the coating amounts to between 0.5 and 10%, and the statistic mean pore size amounts to between 1 and 50 xcexcm, whereby at least nearly exclusively pores with a size of less than 100 xcexcm are present. Further, the pores are stochastically distributed in the surface of the coating, both as far as the area and the size is concerned, the coating comprising a content of bound oxygen of between 0.5 and 8% by weight, and the coating further comprising inclusions of FeO and Fe3O4 crystals, serving as solid lubricants. The method comprises the step of plasma spraying a gas or water atomized coating powder having a particle size of between 5 and 100 xcexcm to the working surface of the cylinder, whereby the spraying distance amounts to between 20 and 50 mm.
The arithmetic mean roughness Ra mentioned in this patent application is sometimes designated simply as xe2x80x9cmean roughness valuexe2x80x9d or as CLA (Center Line Average). It is defined as the height of a rectangle, whose length corresponds to the length of a predetermined measurement path and whose area corresponds to the area between the profile center line and the surface profile. The mean peak-to-valley distance Rz is defined as the mean value of the individual peak-to-valley distances of five consecutive measurement paths (cf. Encyclopedia xe2x80x9cEnzyklopxc3xa4die Naturwissenschaft und Tech-nikxe2x80x9d, Volume 3, Publisher: xe2x80x9cModerne Industriexe2x80x9d, Landsberg a. Lech, Germany 1960, ISBN 3-478-41820-X, Pages 3063 to 3065).
By means of the characteristics according to the invention, on the one hand, it is ensured that enough pores are present for receiving the oil required to form an oil film between piston rings and cylinder wall and, thereby, for keeping the good tribologic properties. On the other hand, due to the very small pores (cavities), the absolute oil consumption can be kept low. In contrast to surface coatings of the working surface of a cylinder according to the prior art, in which the porosity was not or could not be specifically influenced, the surface coating of the invention comprises a porous fundamental structure in which the size of the individual pores is kept within a well defined region. By means of the mechanical finishing, the pores at the surface of the coating are opened.